* {padding:0;margin:0}
html,body {
    height: 100%;
    background-color: pink;

}

.xuetiao {
    position: fixed;
    left: 0px;
    top: 0px;
    width: 100px;
    height: 50px;
    border: solid 5px yellow;
    
}
.xuetiao span {
    width: 100px;
    height: 50px;
    background-color: red;
    display: block;
}